1. Field of the Invention
The present invention relates to a driving unit and a battery-assisted bicycle that assists a rider's pedaling effort with a driving force from an electric motor.
2. Description of the Related Art
There has been a known battery-assisted bicycle that detects a torque generated at a crankshaft rotated together with pedals by a torque detector and controls an electric motor based on a detection result in order to assist the rider's pedaling effort. In such a battery-assisted bicycle, a torque generated at the crankshaft during rotation of the pedals by a rider is detected as a pedaling effort using a tension meter as disclosed in Japanese Patent No. 3412412.
In such a structure adapted to detect a torque generated at the crankshaft, if the torque detector that detects a torque has a failure, appropriate assist control based on the rider's pedaling effort cannot be achieved.
In contrast, there has been a known battery-assisted bicycle that includes a rotation speed sensor adapted to detect a rotation speed of pedals and detects a state of each sensor based on a comparison between a rotation speed sensor signal obtained from the rotation speed sensor and a torque sensor signal obtained from a torque sensor, for example, as disclosed in International Publication No. WO 2011/138138. More specifically, in the disclosed battery-assisted bicycle, a first pedal driving state determined based on a rotation speed sensor signal obtained from the rotation speed sensor for the pedals is compared to a second pedal driving state determined based on a torque sensor signal obtained from the torque sensor, and an error signal is output if they are different.
In the structure disclosed in International Publication WO 2011/138138, an error signal is output if either the torque sensor or the rotation speed sensor experiences a failure. More specifically, the structure disclosed in International Publication WO 2011/138138 allows a failure in the torque sensor and the rotation speed sensor to be detected.
In a battery-assisted bicycle, a pedaling effort acting on a crankshaft and rotation of the crankshaft do not always correspond to each other. In other words, the crankshaft may rotate with no pedaling effort or a pedaling effort may be generated with no rotation of the crankshaft. In the former case, the crankshaft may rotate through a chain in response to rotation of the wheels, for example, on a downslope. In the latter case, a rider may place the feet on the pedals, for example, when the bicycle is in a stationary state.
As described above, depending on the state of the battery-assisted bicycle, a pedal driving state obtained from pedaling effort does not always coincide with a pedal driving state obtained from the crankshaft. Therefore, the failure determination accuracy may be reduced when a failure in each of the sensors is determined based on a determination result indicating an inconsistency between the first pedal driving state obtained from the rotation speed sensor signal related to the rotation speed of the pedals and the second pedal driving state obtained from the torque sensor signal, as disclosed in International Publication No. WO 2011/138138 mentioned above.
As described above, failures in the sensors for use in assist control in a battery-assisted bicycle (such as a torque detector, a crank rotation detector, and a motor rotation detector) cannot be detected with high accuracy using the conventional structure.